TEOCHOPHORE OF HYDEOIDE.S UNCIXATUS (eUPOMATUS) . 561 



anus. The eye-spot is located in a cell in tlie apical region. 

 There is a peri-anal circle of cilia. 



E. B. Wilson (48) hiietiy studied (1890) the segmentation 

 of the egg of a species of Hydroides found at Naples. The 

 order and direction oE the early cleavage planes coincide 

 very closely Avitli those of Eupomatus, and segmentation is 

 of the equal spiral type. I^he spiral symmetry would seem to 

 be retained until a late stage. He did uot definitely observe 

 the cell 4d or follow its history. In his early paper (48) on 

 the origin of the mesoblast bands of Annelids he was of 

 opinion that the bands gave origin to the mesenchyme cells. 

 He did not observe the pole-cells of the bands as described 

 by Hatschek. He pointed out that the head-kidney probably 

 opened into the proctodieam. 



The later development of Psygmobranch us has been 

 studied by Meyer (27), who made some important observa- 

 tions on the mesoderm. He pointed out that in the young 

 trochophore it can be divided under three headings : Eirst, 

 the mesoderm bands, which are closely applied to the ventral 

 surface of the endoderm ; secondly, a collection of irregular 

 cells attached to both ecto- and endoderm, Avhich we can call 

 the embryonic mesenchyme; thirdly, a row of functional 

 primary larval muscles. The mesoderm bands appear as a 

 paired plate of cells converging on one another posteriorly, 

 each ending in a pole-cell — the so-called teloblasts. The 

 plates extend forward into the oral region. The cells of the 

 mesoderm bands can be clearly distinguished from the irre- 

 gular cells of the mesenchyme by their polygonal outlines 

 and their dark-staining nuclei. The larval mesenchyme cells, 

 on the contrary, are irregular in outline, and their nuclei 

 stain less deeply than do those of the bands. The mesen- 

 chyme does not form a compact structure, but is somewhat 

 irregularly arranged into masses on the inner wall of the 

 ectoderm or the wall of the gut. It is divided into a median 

 and a lateral portion, which is again divided into a trunk 

 aud head portion. 



The lateral trunk mesenchyme lies on either side of the 



